A major unsolved problem in human plasma cholesterol metabolism involves the genesis of high density lipoprotein (HDL) and the role which certain small HDL fractions of prebeta electrophoretic mobility play as intermediates of cholesterol transport. These particles may be related to the "nascent" HDL secreted by the liver, and appear to have a unique function in the transfer of cellular cholesterol to the plasma for eventual removal by the liver. The present research includes a detailed analysis of HDL subspecies and seeks a metabolic basis for the complicated heterogeneity observed. Recent research from this laboratory has documented separate pathways for the utilization of cellular and lipoprotein cholesterol within the HDL class of lipoproteins. The current proposal extends this work to an examination of the proteins involved by tracing the movements of apo A-I and apo A-II, the major protein components of HDL. These studies also include an investigation of the interaction of lecithin:cholesterol acyltransferase and the cholesteryl ester transfer protein with the multiple HDL complexes. Epidemiological surveys have documented a strong inverse correlation between the level of HDL in plasma and the prevalence of atherosclerosis, It is critical, therefore, to understand in detail the relationship between HDL and the artery wall, as these lipoprotein complexes probably represent the major mechanism for net removal of cholesterol from the vascular bed. Studies of others have shown that the human arterial wall is unusually enriched in "nascent" HDL, and that HDL pass through the wall into the lymph, instead of being reflected back into plasma. A series of co-culture studies using a multilayer of endothelial and smooth muscle cells, are designed to measure the transport and metabolism of HDL subspecies as they cross the cellular barrier. This will be carried out first, with individual isolated complexes under well-defined conditions followed by experiments in native plasma. A final sequence of studies will examine HDL interactions with the perfused artery wall, using a segment of tissue obtained from surgery. These experiments are aimed at directly measuring the uptake of HDL particles from the luminal medium, their transport through the artery wall, and their appearance on the basal side, possibly enriched in cholesterol.